As in humans, animal knee joints do not include interlocking bones, and instead the femur (thigh bone) and tibia (shin bone) are simply joined with several ligaments—in particular, the cruciate ligaments, which are given that name because they cross from side to side when extending from femur to tibia. Thus, as in humans, animals are also prone to knee injuries owing to damage to their cruciate ligaments. Such injuries, which at best cause diminished mobility and at worst are crippling, are painful for pets and are very distressing for pet owners. In cases where the pet is a companion animal (e.g., a seeing-eye dog), an injury can even have a drastic effect on the pet owner's lifestyle.
Cruciate ligament injuries can be partial tears (disruptions) or complete tears (ruptures). A disruption, while painful, may allow the animal to use the leg, though generally with a limp. However, the joint is still partially unstable: as the animal walks, the femur will slide backwardly over the tibia and generate wear on the joint cartilage, eventually leading to arthritis and greater pain. Additionally, disruptions can grow over time into ruptures. If a cruciate ligament is ruptured, the joint will become entirely unstable because the tibia will no longer be pivotably affixed to the femur, and the animal's ability to walk on the affected leg may be greatly hindered. Disruptions can often be addressed prophylactically via the use of braces and the like, or by surgery. However, a complete rupture can only be addressed by surgery, and recovery is difficult, with arthritis almost invariably developing owing to joint damage.
Since the effects of cruciate ligament injuries can be minimized if they are detected early—as disruptions rather than ruptures—it would be extremely useful to have some means of determining the extent of a ligament injury, whereby the appropriate prophylactic measures can be prescribed before joint damage grows too severe.